Method and coating for protection of molybdenum and its alloys



, March 24, 1959 R. A. LONG 2,878,554

METHODAND COATING FOR PROTECTION OF MOLYBDENUM AND ITS ALLOYS Filed Sept. 16, 1955 INVENT OR an a LONG- ATTORNEY United States Patent METHOD AND COATING FOR PROTECTION OF MOLYBDENUM AND ITS ALLOYS Roger A. Long, Bay Village, Ohio, assignor to Rand Development Corporation, Cleveland, Ohio, a corporation of Ohio Application September 16, 1955, Serial No. 534,771

7 Claims. (Cl. 29198) An object of the present invention is to provide an improved method and coating for the protection of molybdenum and its alloys.

One of the objects of the present inventionis to provide a novel means and method for providing a protective coating on molybdenum or its common alloys so as to protect the same against oxidation and corrosion, and against thermal shock and impact.

Wherever reference is made to molybdenum alloys, it will be understood that the reference is to molybdenum alloys commonly in commercial use where molybdenum is the principal metal in combination with other metals.

One of the objects of the present invention is to provide a protective coating for molybdenum and its alloys comprising approximately 50 to 80 percent nickel, to 40 percent molybdenum and 8 to percent silicon. It is believed that the coating is chiefly Ni MoSi whose composition is approximately 60.6 percent nickel, 24.8 percent molybdenum and 14.5 percent silicon.

Another object of the present invention is to provide a protective coating of molybdenum and its alloys, forming a base, wherein the coating is analloy of nickel, molybdenum and silicon, and wherein the coating is fused to the base. a

A further object of the present invention is the provision of a novel method of protecting a base of molybdenum or its alloys, comprising the forming of a coating consisting mainly of nickel, molybdenum and silicon on the base, and then fusing the coating to the base. V

The invention contemplates various steps for forming the nickel-molybdenum-silicon coating on the base, such as (a) coating the silicon on the base in a vapor phase, then coating the nickel thereover either by electrolytic or vapor process, followed by fusion; (b) co-deposition of nickel and silicon, both in the vapor phase, on the base followed by fusion; (6) mixing nickel and silicon powders in their proper ratio, or mixingnickel, molybdenum and silicon powders in their proper ratio, then coating the same on the base by painting, spraying or dipping, followed by fusion; (:2) forming a nickelmolybdenum-silicon compound, then grinding the same to a fine powder, then coating the same on the base by painting, spraying, or dipping, followed by fusion; and (e) forming an alloy of nickel and silicon in the proper proportions, grinding this alloy to a fine powder, coating said powder onto the base by painting, spraying, or dipping, followed by fusion.

Other objects of the invention include the improving of the nickel-molybdenum-silicon coating by including a small percentage of a metal to increase the fluidity of the coating, by including a small percentage of a metal to increase the oxidation resistance of the coating, or by including a small percentage of a metal to increase the toughness or other desirable characteristics of the coating as will hereinafter appear.

Other objects and advantages of the invention will appear from the drawings and description and the essen 2 tial features thereof will be set forth in the appended claims.

The drawing is a photo-micrograph of a cross-section through a molybdenum base having a nickel-molybdenumsilicon coating, magnified one thousand times.

A specific application of the present invention is the protection of gas turbine blades and other parts, working at elevated temperature up to approximately 2000 degrees F. The invention has other uses in connection with heating elements, heat shields for guided missiles, flame holders, etc. An object of the present invention, therefore, is to obtain a uniform coating of even thickness, but still thin enough to withstand fatigue action Without spalling or flaking, and thin enough so that it is not expensive. It must be capable of covering various shapes. The coating must have good oxidation resistance, and resistance to fuel additives, lead oxide, sulphur, and other corrosive atmospheres. The coating must have good thermal shock resistance, which means that its thermal expansion and specific heat must be similar to that of molybdenum, upon which it is coated. It must expand and contract with the molybdenum. Impact resistance is most important in gas turbine blades, therefore, the coating must Withstand particle impact at high velocities. The coating must have a toughness and ductility to resist the shock impact and to deform without breaking or spalling under such impact. Bond strength between the coating and the base metal is, of course, one of the most important characteristics of the coating, and the fact that it is a fused coating is very important to this bond strength. The resistance to erosion is important because of the dust, dirt and carbon particles which flow through gas turbines. Also, the fact that it is a thin coating, and therefore, will withstand flexing without. flaking, is important.

One of the novel features of the present coating is that, whereas molybdenum disilicide melts at 3600 degrees F. and nickel melts at 2600 degrees F., the chemical composition of the present coating mixture, probably Ni MoSi melts at approximately 2100 degrees F. This enables the present coating to be applied first to the surface of molybdenum and thereafter fused to the molybdenum base at a temperature not over approximately 2100 degrees F. It is well known that molybdenum and the common molybdenum alloys presently in commercial use may be heated to 2100 degrees F. for a short time without recrystallization of the base. Thus, the strength properties of the base material remain high.

Several methods are herein disclosed for applying my improved protective coating to the base. In some cases, the molybdenum content of the coating is taken from the base .metal, while in other methods, the molybdenum is supplied in the coating itself independently of the base metal. In each case, however, I end up with a nickelmolybdenum-silicon coating which has a fused bond to the base of molybdenum or molybdenum alloy.

This is clearly seen in the drawing which is a crosssection through molybdenum base having my novel protective coating and wherein the zone marked B is the molybdenum base, the zone marked C is the coating per se, and the zone marked Fis the fusion bond between the coating and the base.

One method by which the coating may be formed is to first coat the molybdenum or molybdenum alloy base with silicon by one of the standard processes. One such currently used process comprises heating the base to an elevated temperature after which silicon tetrachloride vapor is passed over the heated surface so as to form a coating comprising silicon and molybdenum in combination. The layer nearest the base is MoSi. The outer layer is MoSi This coating is preferably kept to a thick- 3 j ness from 0.0005 inch to 0.0015 inch. It is believed that onethousandth of an inch thickness is probably'sufficient. The coated silicon part is then removed and is prepared forstandard Watts' nickel plating or electroless nickel plating, or nickel vapor deposition by the carbonyl process. Nickel is plated onto the piece over the siliconmolybdenum coating to a'thickness of approximately 0.0005'to 0.001 inch. Too muchnickel is detrimental during the fusion step which follows because it causes an uneven and uncontrollable flow of the coating over the base. The next step is to heat the coated part in a non-oxidizing atmosphere to a temperature at which the surface alloy melts and flows over the entire surface. I

prefer to heat the piece in a furnace in a hydrogen at-.

mosphere or in a vacuum, preferably the latter. The temperature in the furnace should not be allowed to rise substantially over 2100 degrees F. as the molybdenum base will not stand this temperature for very long and will not stand a higher temperature without the danger of recrystallization. The melting point of Ni Mosi is approximately 2100 degrees F. and when the surface coating is seen to be flowing, this indicates the formation of the low meltingnickel-molybdenum-silicon alloy of the present invention. After suflicient time is allowed for complete melting of the surface coating material, the piece is cooled which results in solidification and coating of the base material.

Another method of providing the coating on the molybdenum or molybdenum alloy base is to heat the base and then deposit both nickel and silicon at the same time from the vapor state onto the heated molybdenum surface, resulting in the formation of the nickel molybdenumsilicide. After depositionof this coating, the piece is put through the same fusion process as first described by heating the same in a non-oxidizing atmosphere to a point not over 2100 degrees F. until the surface coating has melted and fused to'the base material.

' Another method of forming the coating on the molybdenum base is to first mix finely powdered nickel and finely powdered silicon in the ratio of approximately 4.2 parts of nickel to one part of silicon, and then dispersing these mixed powders in a disappearing vehicle for painting, spraying or dipping the coating on the surface of the base metal to be coated. By disappearing vehicle, I mean any vehicle which will hold the coating in place for the remainder of the process but which will leave practically no traces in the finished product. For instance, a synthetic plastic material which will disappear 1y of Ni MoSi, is then dispersed in a disappearing vehicle andis's'prayed,"painted or dipped on the surface of molybdenum or molybdenum alloy base to be coated..

nickel silicide is then finely divided and dispersed in a disappearing vehicle, after which it is coated on the molybdenum base by painting, spraying or dipping. This is followed by the fusion step of heating the part in anonoxidizing atmosphere in a furnace to a temperature not substantially above 2100 degrees F. until the coating is:

melted and covers the entire surface and is fused to the base. 1

In each case, where the component molybdenum is left out of the coating as first applied to the base, the nickel and silicon will pick up the molybdenum from the base metal so that in each case I end up with a nickel-molybq v denum-silicon coating in the finished product.

in the heating of the fusion step will be satisfactory. The

coated part is then heated to provide the fusion step as by heating in a non-oxidizing atmosphere in a furnace to a temperature not substantially above 2100 degrees F. until the coating has melted and flowed over the entire surface and fused to the base. In this process, the nickel and silicon will pick up molybdenum from the base.

A further method slightly different from that mentioned in the preceding paragraph comprises mixing finely divided powders of nickel, molybdenum and silicon in the ratio of approximately 61 parts of nickel to 25 parts of molybdenum and 14 parts of silicon. These mixed powders are thendispersed in a disappearing vehicle as described above and then painted, sprayed or dipped on the surface of the base metal to be coated. After that the fusion step occurs by heating the product in a nonoxidizing atmosphere to a temperature not substantially above 2100 degrees F. until the coating has melted and flowed over the entire'surface and fused to the base.

A further method of providing the desired coating is to first make up an alloy of approximately the composition Ni MoSi casting the alloy and grinding the cast material then to a fine 'particle'subdivision. Otherwise this alloy may be made by hydride processes so as to provide the; alloy of-the desired composition as set forth. Such a previously provided finely. divided powder consisting main:

heals itself.

While I'have described the preferred method of fusing the coating to the base by heating in a furnace as above.

described, it will be understood that I intend to include equivalent electrical heating means such as induction heating, high frequency heating, etc.

In addition to the methods described above, the use of additional elements for enhancing certain properties of the coating is desirable. For example, the coating nickelm'olybdenum-silicon might not be as fluid as one would like. It would then be desirable to add an additional ele ment to increase the fluidity of this alloy. Small amounts 7 of iron, manganese or cobalt may be added for this purtougher than that of the nickel-molybdenum-silicon mentioned above. In such cases, a small amount of another metal may be added to increase the toughness of the coat ing. Small amounts of vanadium or manganese may be utilized for this purpose.

'One of the characteristics of the nickel-molybdenum-; silicon coating is its self-healing properties. Even though the coating be perfect when first formed, pores of microscop lc size may be formed after the product has gone into use.

molybdenum trioxide.

after, the silicon in the coating will stabilize the nickel:

molybdate so'that it will not spall upon heating and cool- 7 ing. This, then, provides a stable nickel molybdate material in the pore which in itself is oxidation resistant and will prevent further oxidation of the underlying molyb-f denum metal at that particular pore.

What I, claim is: r 1. A protective coating on a base consisting essentially of molybdenum comprising approximately 50 percent to percent 'nickel, 15 percent to 40 percent molybdenum g and 8 percent to 20 percent silicon including a fusion zone at the interface between said coating and base where the Y coating and base are partly fused together.

2. The method of coatinga base consisting essentially of molybdenum, comprising mixing finely divided nickel and molybdenum and silicon powders in the ratio of ap- If such a microscopic pore should develop through the coating to the base, then molybdenum in the base starts to oxidize forming a molybdenum oxide or Then the nickel available in my coating will form a nickel molybdate by diffusion. There- Thus the coating pearing vehicle onto said base in a thin layer, and then fusing said layer to said base.

3. The method of coating :1 base consisting essentially of molybdenum, comprising making an alloy comprising approximately 61 percent nickel, 25 percent molybdenum and 14 percent silicon, grinding said alloy to a fine powder, coating said powder in a disappearing vehicle onto said base in a thin layer, and then fusing said layer to said base.

4. The method of coating a base consisting essentially of molybdenum, comprising making an alloy comprising approximately 61 percent nickel, 25 percent molybdenum and 14 percent silicon, grinding said alloy to a fine powder, coating said powder in a disappearing vehicle onto said base in a thin layer, and then fusing said layer to said base at a temperature not substantially in excess of 2100 degrees F. in a non-oxidizing atmosphere.

5. An article comprising a structure consisting essentially of molybdenum and having fused thereon a protective coating comprising approximately 61 percent nickel,

percent molybdenum and 14 percent silicon.

-6. An article comprising a structure consisting essentially of molybdenum and having fused thereon a protec- 6 tive coating comprising approximately percent to percent nickel, 15 percent to 40 percent molybdenum and 8 to 20 percent silicon.

7. An article comprising a structure consisting essentially of molybdenum and having fused thereon a protective coating consisting mainly of approximately 60.7 percent nickel, 24.8 percent molybdenum and 14.5 percent silicon.

References Cited in the file of this patent UNITED STATES PATENTS 2,685,124 Toulmin et al Aug. 3, 1954 2,711,973 Wainer et a1 June 28, 1955 FOREIGN PATENTS 716,604 Great Britain Oct. 13, 1954 723,899 Great Britain Feb. 6, 1955 OTHER REFERENCES Metal Treatment, August 1953, pages 341-345. Mellor: Comprehensive Treatise on Inorganic and Theoretical Chemistry, vol. 15, page 247. published 1936. 

1. A PROTECTIVE COATING ON A BASE CONSISTING ESSENTIALLY OF MOLYBDENUM COMPRISING APPROXIMATELY 50 PERCENT TO 80 PERCENT NICKEL. 15 PERCENT TO 40 PERCENT MOLYBDENUM AND 8 PERCENT TO 20 PERCENT SILICON INCLUDING A FUSION ZONE AT THE INETERFACE BETWEEN SAID COATING AND BASE WHERE THE COATING AND BASE ARE PARTLY FUSED TOGETHER. 